Accounting for periods of wetness in displacement of 'Fusarium pseudograminearum' from cereal straw

Displacement of pathogenic 'Fusarium' species from cereal residues by other fungi is an important mechanism for the effectiveness of fallows and crop rotations on disease management, as well as in potential biological control. The effect of fluctuating environmental conditions on the rate of displacement was assessed using two different approaches. In the first, wetness durations between 4 and 10 h were simulated by spraying water onto straw inoculated with 'Fusarium pseudograminearum' and antagonists in a greenhouse. For a given cumulative period of wetness, displacement of 'F. pseudograminearum' was generally higher for short (4 h) than longer (10 h) wetting durations, indicating that it was the number of wetting events, rather than their individual durations, that determined the rate of displacement. In the second approach, exponential decay models using thermal time adjusted for rainfall were fitted to published data on survival of Fusarium species in residues. Heat sums calculated from the mean temperature of days on which rain fell, or rainday-degrees (RDD), gave good fits to data from short-term experiments on displacement of 'F. pseudograminearum' by antagonists under natural conditions. RDD and two other indices, decomposition days (DCD) and corrected degree-days (CDD), were equally satisfactory for modelling straw decomposition and mortality of 'Fusarium' in longer term data sets. Such models could be useful for predicting the effects of environmental variation on rotations and biocontrol for 'Fusarium' management in cereals.